Good point OrphanCrow, that's one I always wondered about too. I can't see any justification for not using Jehovah in Heb 1:10 other than it's embarrassing for JWs to explain why Jesus is apparently called Jehovah here.
There might possibly be an issue with the Hebrew, inasmuch as I'm not sure if the divine name is used in this Psalm in Hebrew in the places where the later Christian LXX has Lord.
But I don't think they should be afraid of translating such verses that apply the name Jehovah to Jesus, because the NT says that Jesus is given the name above every name. (Phil 2) In other words the name of God is given to Jesus and he acts as Jehovah's representative. It was customary for a representative of the monarch to be addressed as the monarch, and this is the scenario that is applied to Jesus. Plus another angle is that Hebrews 1 isn't necessarily identifying Jesus with Jehovah in Heb 1:10 any more than, strictly speaking, it is identifying Jesus with Solomon(?) in Heb 1:8. Because the phrase it uses is "with respect to the son" rather than addressing the son. Indicating it's aspects of the Son's character rather than an identification that is in focus.
But I agree, if being consistent, Heb 1:10 should probably use God's name even if the extant Hebrew doesn't have the Tetragram, because the assumption should reasonably be that the LXX was based on a Hebrew text that did contain the name, and that the early LXX would have used the name.
There are a few other such examples but this is a good example.